Conventionally, as a system to reuse a sheet, an inkjet printer and so on are proposed which prints an ink including the leuco dye by an inkjet apparatus and erases using a thermal bar or a thermal head.
In Japanese Patent Application Publication No. 2001-302954, an ink is disclosed which exhibits colors by the chemical combination of the coloring substance and the developer at a normal temperature equal to or lower than 40° C., the chemical combination breaks down at a temperature equal to or larger than 100° C., and is made colorless by the chemical combination of the developer and the decolorizing agent. In addition, in Japanese Patent Application Publication No. 2001-302954, an inkjet printer using the ink and further an inkjet printer for both the recording and erasing use provided with a heating mechanism for erasing are disclosed.
In addition, in Japanese Patent Application Publication No. 2005-89548, an ink material having a hysteresis property in the coloring and decolorizing curve in order to fix the coloring and decolorizing at the room temperature.
However, in case that the sheet is to be reused which the image is formed on and decolorized from using the ink as disclosed in Japanese Patent Application Publications No. 2001-302954 and No. 2005-89548, the ink might exhibit colors again when the sheet is cooled. This is because the pigment and the developer once separated off by heating recombines under the circumstance equal to or lower than a prescribed temperature that is lower than the normal temperature. That is, the image formed in past times can made be reappeared intentionally by cooling the sheet after erasing. Accordingly, the content erased for the reuse might be leaked to the third party, causing a great problem in security.